2. Evapotranspiration
Alec
● Describes return of water to the atmosphere
○ By transpiration from plants
○ And evaporation from surrounding
ground and/or bodies of water
● Is a more comprehensive measurement of
water loss from crops
○ Crops utilize water that is inside them
AND water contained in the surrounding
soils
● Widely used in irrigation research
Evaporation + Transpiration =
Evapotranspiration
3. Lake Murray, SC
● This project sought to estimate evapotranspiration
rates by using known reservoir evaporation rates
● Chose Lake Murray, a reservoir close to Clemson!
○ Centrally located in SC
○ 5th largest lake in SC by area
○ Became a lake when dammed in 1930
○ Was the world’s largest man-made reservoir
when made
● Largest privately owned reservoir in the United
States
○ South Carolina Electric & Gas Co.
4. Lake Murray, SC
Drainage Area
[sq mi]
Surface Area
[acres]
Dead
[acre-ft]
Total
[acre-ft]
Usable
[acre-ft]
2,420 50,800 500,000 2,114,000 1,614,000
Table 1: Drainage and Storage Areas
Data taken from USGS Water Supply Report 1838:
“Reservoirs in the United States” (1954)
5. Part I - Pan Evaporation Drew
● Part I uses “Pan Evaporation” data
○ The standard for measuring evaporation
rates
○ Record daily change in level of water
○ Record daily rainfall
○ Refill at end of the day
● Distinguishes evaporation rates based
on weather factors and is simple
● Class A Pan
○ Typically installed on a raised wooden
platform to allow air to freely circulate
○ Diameter = 47.5 in
○ Depth = 10 in “Class A” Pan
6. Part I - Annual Lake Evaporation
● Class A Pan data gathered from:
“Evaporation Maps for the U.S” (1959)
● Plate 2:
○ Annual avg. lake evaporation = 42 in/yr
○ Annual Vol. evaporated from Lake Murray:
■ 42 in x 50,800 acres = 2.134 x 10⁶ acre-in
■ 3.5 ft x 50,800 acres = 1.778 x 10⁵ acre-ft
■ 1.07 m x 2.056 x 10⁸ m²= 2.193 x 10⁸ m³
● Plate 1:
○ Annual avg. pan evaporation = 55 in/yr
● Plate 3:
○ Pan coefficient = 0.75 → 55 in x 0.75 = 41.75 in/yr (approx. avg. annual lake evaporation)
Example of Plate Data
7. Part I - Annual Lake Evaporation
● “Stress Period” → May - October
○ Generally, most evaporation occurs during this time
● Plate 4:
○ Plate 4: 67% of evaporation during stress period
○ 42 in/yr x 0.67 = 28.14 in during stress period
● Stress Distribution:
○ Northern pans ~ 80%
○ Southern pans ~ 60%
○ Colder climates lose more water during hot months
○ Warmer climates lose less, because they tend to be hot year-round
Stress Distribution
8. Part I - Annual Lake Evaporation
● Plate 5:
○ Pan Standard deviation = 3.5 in
○ Lake S.D. = 3.5 in x 0.75 = 2.625 in
○ Expect annual evaporation to be between:
■ 68% Confidence Interval: (39.4 - 44.6 in)
■ 95% Confidence Interval: (36.8 - 47.3 in)
■ 99.7% Confidence Interval: (34.1 - 49.9 in)
● Coefficient of Variation: [SD / mean]
○ At Lake Murray = 3.5/42 = 0.083
○ Ratios tend to drop as one heads towards the milder/stable coastal climates (smaller
S.D.) and stay high in central continental climates (larger S.D.)
Standard Deviations across the US
9. Part II - Nomograph Evan
● Allows us to estimate
daily pan evaporation
based on certain climatic
data
● Means the pan
evaporation can be
estimated for anywhere
in the US!
● Estimates based on
“average day of average
month”
Wind
Movement
[mi/day]
Avg. Dew
Point Temp
[deg F]
Solar
Radiatio
n
[Langley/
day]
Daily Pan
Evaporation
[.01 in]
***This is what
we’re solving for!
10. Part II - Daily/Monthly Lake Evaporation
● We obtain all this data from:
“Climatic Atlas of the United States” (1968)
● Assembled and written by:
○ National Oceanic and Atmospheric
Administration (NOAA)
○ Environmental Science Services Administration
(ESSA)
■ Disbanded in 1970
■ Has an interesting logo:
11. Part II - Daily/Monthly Lake Evaporation
● The Atlas presents data by
month
● Any month can be picked for
nomograph
○ We picked 3 months
○ Allows understanding of evaporation
throughout the year
● Mean Daily Air Temperature:
○ April = 62 deg F
○ July = 80 deg F
○ November = 54 deg F
12. Part II - Daily/Monthly Lake Evaporation
● Solar Radiation
○ April = 505 Langleys
○ July = 530 Langleys
○ November = 268 Langleys
→ 1 Langley/day = 0.484583 Watt/m2
→ Means: 505 Langleys = 244.7 W/m2
13. Part II - Daily/Monthly Lake Evaporation
● Dew Point:
○ April: 48 deg F
○ July: 69 deg F
○ November: 40 deg F
● Dew Point is the temperature
which air must be cooled to in
order to be saturated with
water vapor
○ In the summer, high dew points
means it is more likely to be humid!
14. Part II - Daily/Monthly Lake Evaporation
● Wind Movement
○ April: 9 mph
○ July: 7 mph
○ November: 6 mph
● When put into miles/day:
○ April: 216 mpd
○ July: 168 mpd
○ November: 144 mpd
15. Part II - Nomograph Values
● Each of the values discussed plays
a role in how much water is
evaporated
● The nomograph is read from both
clockwise & counterclockwise
April July November
Evap:
[in/day]
0.31 0.34 0.20
X
Y
16. Part III - Estimating
Evapotranspiration
● Radiation, wind,
humidity, and crop
cover are ignored
● Temperature,
precipitation,
streamflow (runoff),
groundwater flow, and
soil water content are
used for estimation
Rose
17. Part III - The Water Balance Equation
● Thornthwaite’s Equation
○ Underestimates ET by a
factor of 1.3 to 2 in arid
climates
○ Independent of humidity,
radiation, and wind
○ Should be used as a modifier
19. Part III - The Water Balance Equation
● Blaney-Criddle Equation
○ Precipitation, runoff,
groundwater flow, and soil water
content are taken into account
○ Groundwater inflow and outflow
and soil water change are
constant over long periods of
time
20. Conclusions
Freshwater Surface
Evaporation
Thornthwaite’s PET Blaney-Criddle ABT
39.4 - 44.6 in/yr 36-37 in/yr 28.32 in/yr
● Free water surface
that has water
readily available for
ET
● May overestimate
PET in hot climates
● Upper-bound ET
● Only relevant to dry
parts of the
designated area
21. Further Studies
● Penman-Monteith
Equation
(Simplified Version)
● λE= evaporative latent heat flux (MJ m-2 d-1)
● Δ= slope of the saturated vapor pressure
curve
● Rn = net radiation flux (MJ m-2 d-1)
● G = sensible heat flux into the soil (MJ m-2d-1)
● ³ = psychrometric constant (kPa °C-1)
● Ea = vapor transport of flux (mm d-1)
Editor's Notes
“DEAD STORAGE Dead storage is the volume, in acre-feet, below the lowest controllable pool level.
TOTAL STORAGE Total storage is the volume, in acre-feet, below the maximum controllable pool level. For a dam having an ungated overflow spillway, it is the total volume below the spillway crest. When there is flow over the spillway, the total volume of stored water exceeds the total reservoir capacity, but such uncontrollable excess storage is excluded from this compilation.
USABLE STORAGE The usable storage is the volume, in acre-feet, normally available for release from a reservoir below the maximum controllable level. For power and irrigation reservoirs, this definition is adequate. For a flood-control reservoir the volume above the spillway crest is excluded from the usable storage although recognized and allowed for in the design of the dam and reservoir. For multipurpose reservoirs the volume available for release may be dictated by allocations for various uses, but such complications have not been considered in this report.”
From USGS Water Supply Report 1838
“This level of confidence can lead to some confusion. It is not a statement about the sampling procedure or population. Instead, it is giving an indication of the success of the process of construction of a confidence interval. For example, confidence intervals with confidence of 80 percent will, in the long run, miss the true population parameter one out of every five times.”
https://www.thoughtco.com/what-is-a-confidence-interval-3126415
“I” is derived from a mean of monthly temperatures and alpha is a constant dependent on I
This chart of Blackwell, South Carolina’s temperature and precipitation demonstrates why Thornthwaite’s estimations fall short by not considering precipitation in PET estimations.
South Carolina happens to be a very hot climate, so Thornthwaite’s temperature dependent equation can be expected to over-predict PET. However, PET refers to an upper limit of ET. Therefore Blaney-Criddle and Thornthwaite may both be feasible.
